Researchers in California have successfully predicted rates of disease in plants living in grassland ecosystems. The researchers said more common plants in these ecosystems were much more susceptible to disease.

Published on Earth Day (Wednesday 22nd April), a study of disease in a California grassland has revealed fundamental principles underlying the spread of pathogens - disease-causing microbes - among species.

Published in the journal Nature, the results have implications for maintaining biodiversity and for addressing practical plant disease problems.

Researchers at the University of California, Santa Cruz, studied the phenomenon of "pathogen spillover" in grassland species on the UC Santa Cruz campus. Pathogen spillover occurs when disease from one species causes an epidemic in another. One example is bats transmitting Ebola to humans.

The Santa Cruz researchers found that the amount of disease present on each species could be predicted by the abundance of its close relatives in the grassland. When there were many individuals of the same or similar species living close together, pathogens spread more quickly.

Perhaps unexpectedly, the pattern of disease spread in turn promotes biodiversity by creating openings for less common species that are not attacked by the same pathogens as the more common species.

Predicting disease prevalence

The findings revealed a tight link between the structure of a plant community and the vulnerability of individual species to disease.

"These scientists demonstrate that the relatedness of species in communities is an important predictor of disease prevalence," said Alan Tessier, acting director of the National Science Foundation's (NSF) Division of Environmental Biology, which funded the research.

The researchers were able to predict which plant species introduced into the grassland would be most strongly affected by naturally-occurring diseases.

Ingrid Parker, an ecologist and evolutionary biologist at UC Santa Cruz and first author of the paper, said the study adds an important new dimension to a longstanding concept in ecology known as the "rare species advantage."

"The rare species advantage is thought to be a major driver of biodiversity in natural ecosystems," Parker said. "Most pathogens are not host specialists - they can easily move from one species to another. Whether pathogens 'spill over' depends on how closely related other species nearby are.

"Our study shows that it's the structure of the whole community around a species that affects its vulnerability to disease."

Large-scale experiment

In a large-scale experiment, the researchers introduced 44 plant species from outside California to the grassland system. (The plants were removed before they reproduced.)

The biologists found that species with few close relatives in the grassland escaped disease, while those closely related to many resident species always showed high levels of disease.

The researchers were able to make surprisingly accurate predictions of disease in introduced species based on their phylogenetic - or evolutionary - distance from local species.

Parker said "It was kind of shocking how well we were able to predict disease at a local scale.”

To aid their predictions of disease dynamics, the researchers used a model developed by scientist Gregory Gilbert at UC Santa Cruz and colleagues who work with the U.S. Department of Agriculture's (USDA) Animal and Plant Health Inspection Service. The model is based on USDA's global database of fungal pathogens and host plants, and can be used to predict the probability of two species sharing a pathogen based on how closely related they are.

"If a plant pathogen from Brazil suddenly shows up in southern California, you want to know what plants in California are most likely to be attacked," Gilbert said.

By showing that Gilbert’s pathogen model makes accurate predictions, the researchers said their results suggest a range of potential applications.

The model - called the PhyloSusceptibility model - could help avoid disease problems affecting proposed horticultural imports or reforestation projects. It could also be used in agriculture to design intercropping or rotation systems to decrease crop disease.

Vulnerability of local species to "pathogen spillover"

Imported plants can bring new pathogens and pests into an area. The PhyloSusceptibility model could be used to assess the vulnerability of local species to pathogen spillover from such plant introductions, the scientists say.

While the PhyloSusceptibility model used in this study was based on data for fungal pathogens, Gilbert said the team has also created versions based on data for eight other groups of pests and pathogens, including insects, nematodes, bacteria and viruses.